The motor speed control apparatus of this type is shown in FIG. 7. As shown, a DC motor 3 and a thyristor 4 are connected to the DC output of a full-wave rectifier 2 which is connected to the terminals of an AC mains supply 1. Across the terminals of the thyristor 4 is connected a resistor 5 and a capacitor 6 in a series circuit so that when the thyristor 4 is in a turned-off state, the capacitor 5 is charged through the resistor 5. Capacitor 6 is discharged through a bidirectional breakdown diode, or diac 7 which is coupled between the gate of thyristor 4 and one end of the capacitor 6 when the voltage across the capacitor 6 reaches a predetermined level.
More specifically, if the anode of thyristor 4, shown at A in FIG. 7, is impressed with a voltage having a substantially sinusoidal waveform as shown in FIG. 8, the voltage of the capacitor 6 developed at point B increases gradually and the bidirectional breakdown diode 7 is turned on when it reaches its breakdown voltage V.sub.B1 at time T.sub.1, turning the thyristor 4 on to control the phase of the motor 3.
In this way, the capacitor 6 is charged with a voltage which is equal to the difference between the mains supply voltage and the counter EMF (electromotive force) generated in the motor 3 so that it operates at a low speed. Under this low speed condition, the prior art apparatus operates to maintain the speed of the motor 3 constant under varying torque.
However, the prior art apparatus suffers from speed instability when the motor is operated at a still lower speed. Assume that the breakdown voltage of the diac 7 is raised V.sub.B1 to to V.sub.B2 as shown in FIG. 8 in an attempt to reduce the motor speed, the rate of voltage increase at point B decreases gradually and the breakdown occurs at time T.sub.2. Thus, the increase in breakdown voltage results in a disproportionally large shift in breakdown time. This implies that the motor would suffer speed instability if an attempt is made to lower the motor operating speed by merely increasing the breakdown voltage.
Furthermore, if the mains supply voltage has increased, the voltage at point A increases, causing the charging time to reduce with a resultant decrease in trigger intervals. Thus, the motor speed increases with an increase in mains supply voltage.